The Liquid Crystal Display (LCD) possesses advantages of thin body, power saving and no radiation to be widely used in many application scope. Such as LCD TV, mobile phone, personal digital assistant (PDA), digital camera, notebook, laptop, and dominates the flat panel display field.
In recent years, the Thin Film Transistor-LCD (TFT-LCD) has been rapidly developed and applied widely. For the TFT-LCD in the mainstream market, three types, which respectively are Twisted Nematic (TN), Super Twisted Nematic (STN), In-Plane Switching (IPS) and Vertical Alignment (VA) can be illustrated. The VA liquid crystal display possesses extremely high contrast than the liquid crystal displays of other types, which can reach up to 4000-8000 in general. It has very wide application in large scale display, such as television or etc.
The reason why the VA liquid crystal display possesses extremely high contrast is that the liquid crystal molecules are vertically aligned to the substrate surface, and no phase difference exists, and light leakage is very small, and the dark state brightness is extremely small at the dark state without applying electricity. The lower the brightness at the dark state can be, the higher the contrast is according to the contrast calculation formula. For vertically aligning the liquid crystal molecules of the VA liquid crystal display to the substrate surface, it is demanded to implement vertical alignment treatment to the liquid crystal molecules. The most common way is to coat vertical alignment solution (Polyimide, PI solution) on specific areas of surfaces of the upper substrate and the lower substrate, and then to bake the substrates for a long period in certain temperature for curing the solvent in the alignment solution. Thus, PI alignment layers are formed on the surfaces of the glass substrates. As shown in FIG. 1, the traditional VA liquid crystal display comprises an upper glass substrate 10, a lower glass substrate 20 oppositely located to the upper glass substrate 10, a liquid crystal layer 40 sandwiched between the upper glass substrate 10 and the lower glass substrate 20, PI alignment layers 30 formed on a surface of the upper glass substrate 10 facing downward to the glass substrate 20 and a surface of the lower glass substrate 20 facing upward to the upper glass substrate 10. However, because the VA liquid crystal display utilizes vertical twist liquid crystals and the birefraction difference of the liquid crystal molecules is larger, the issue of the color shift under large view angle is more serious.
For earning better wide view angle property for the VA liquid crystal display panel to improve the color shift issue, the multi-domain VA (MVA) technology is commonly utilized, which is to divide a sub pixel into many districts and drive the liquid crystals in respective districts to lie down toward different directions as applying voltage. Thus, the watch results from respective directions can be equal. There are many ways to achieve MVA technology. Please refer to FIG. 2 and FIG. 3. One of these ways is to process the ITO pixel electrode 70 at one side to be a pozidriv slit pattern, and the common electrode 80 is a plane electrode. With the special ITO pixel electrode pattern, the tilt electric field can induce the liquid crystal molecules 40 in different areas to lie down toward different directions. FIG. 2 is a top view diagram of one side of a lower substrate 20 in an MVA type liquid crystal display panel. 210 and 220 respectively are a scan line and a data line. One sub pixel is divided into four areas. The ITO pixel electrode 70 in each area forms pixel electrode branches and a slit interval pattern extending toward different directions. FIG. 3 is a sectional diagram of an MVA type liquid crystal display panel corresponding to A-A portion shown in FIG. 2. The pixel electrode 70 having slits is formed on a flat lower passivation layer 60, and the surface of the PI alignment layer 30 covering the pixel electrode 70 is irregular. However, the plane common electrode 80 is formed on a flat upper passivation layer 90, and the surface of the PI alignment layer 30 covering the plane common electrode 80 appears to be a plane surface. Therefore, different cell gaps exist in the slit corresponding areas and the pixel electrode branch corresponding areas.
According to the transmittance formula of the VA liquid crystal display panel:
                    T        =                              1            2                    ⁢                      sin            2                    ⁢          2          ⁢          Δ          ⁢                                          ⁢                      Φsin            2                    ⁢                      Γ            2                                              (        1        )            wherein T is transmittance, and Δϕ is the included angle between the long axis of the liquid crystal and the polarizer, of which the efficiency is the maximum as the angle is 45°; ┌ is the phase difference, and the calculation formula is 2π*Δn*d/λ, and d is the cell gap, and Δn is the refractivity difference of the long, short axes of the liquid crystal; as known according to formula (1), as ┌ is π, the transmittance is the maximum, i.e. as d=λ/(2*Δn), the transmittance is the maximum. Too large or too small cell gap can cause the reduction of the transmittance.
As shown in FIG. 3, the transmittance of the pixel electrode branch corresponding area is designed to be the maximum in general, and the cell gap D′ of the slit corresponding area is different from the cell gap D of the pixel electrode branch corresponding area and the transmittance of the liquid crystal display panel will decrease, accordingly; besides, no ITO pixel electrode exists in the slit corresponding area, and in comparison with the pixel electrode branch corresponding area, the electrical field of the slit corresponding area is weaker, and the tilt degree of the liquid crystal is smaller. Thus, the transmittance of the slit corresponding area drops. Ultimately, the display brightness of the slit corresponding area is darker to result in nonuniform display brightness of the liquid crystal display panel.